This invention relates to electrography and more particularly it relates to magnetic carrier particles and developers used for the dry development of electrostatic charge images.
In electrography, an electrostatic charge image is formed on a dielectric surface, typically the surface of the photoconductive recording element. Development of this image is typically achieved by contacting it with a two-component developer comprising a mixture of pigmented resinous particles, known as toner, and magnetically attractable particles, known as carrier. The carrier particles serve as sites against which the non-magnetic toner particles can impinge and thereby acquire a triboelectric charge opposite to that of the electrostatic image. During contact between the electrostatic image and the developer mixture, the toner particles are stripped from the carrier particles to which they had formerly adhered (via triboelectric forces) by the relatively strong electrostatic forces associated with the charge image. In this manner, the toner particles are deposited on the electrostatic image to render it visible.
It is generally known to apply developer compositions of the above type to electrostatic images by means of a magnetic applicator, also known as a magnetic brush, which comprises a cylindrical sleeve of non-magnetic material having a magnetic core positioned therein. The core usually comprises a plurality of parallel magnetic strips arranged around the core surface to present alternating north and south oriented magnetic fields. These fields project radially, through the sleeve, and serve to attract the developer composition to the sleeve outer surface to form what is commonly referred to in the art as a xe2x80x9cbrushxe2x80x9d or xe2x80x9cnapxe2x80x9d. Either or both the cylindrical sleeve and the magnetic core are rotated with respect to each other to cause the developer to advance from a supply sump to a position in which it contacts the electrostatic image to be developed. After development, the toner depleted carrier particles are returned to the sump for toner replenishment.
Conventionally, carrier particles made of soft magnetic materials have been employed to carry and deliver the toner particles to the electrostatic image. U.S. Pat. Nos. 4,546,060, 4,473,029 and 5,376,492, the teachings of which are incorporated herein by reference in their entirety, teach use of hard magnetic materials as carrier particles and also apparatus for development of electrostatic images utilizing such hard magnetic carrier particles. These patents require that the carrier particles comprise a hard magnetic material exhibiting a coercivity of at least 300 Oersteds when magnetically saturated and an induced magnetic moment of at least 20 EMU/gm when in an applied magnetic field of 1000 Oersteds. The terms xe2x80x9chardxe2x80x9d and xe2x80x9csoftxe2x80x9d when referring to magnetic materials have the generally accepted meaning as indicated on page 18 of Introduction To Magnetic Materials by B. D. Cullity published by Addison-Wesley Publishing Company, 1972. These hard magnetic carrier materials represent a great advance over the use of soft magnetic carrier materials in that the speed of development is remarkably increased with good image development. Speeds as high as four times the maximum speed utilized in the use of soft magnetic carrier particles have been demonstrated.
In the methods taught by the foregoing patents, the developer is moved in the same direction as the electrostatic image to be developed by high-speed rotation of the multi-pole magnetic core within the sleeve, with the developer being disposed on the outer surface of the sleeve. Rapid pole transitions on the sleeve are mechanically resisted by the carrier because of its high coercivity. The nap, also called xe2x80x9cstringsxe2x80x9d or xe2x80x9cchainsxe2x80x9d, of carrier (with toner particles disposed on the surface of the carrier particles), rapidly xe2x80x9cflipsxe2x80x9d on the sleeve in order to align with the magnetic field reversals imposed by the rotating magnetic core, and as a result, moves with the toner on the sleeve through the development zone in contact with or close relation to the electrostatic image on a photoconductor. This interaction of the developer with the charge image is referred to as xe2x80x9ccontactxe2x80x9d or xe2x80x9ccontactingxe2x80x9d herein for purposes of convenience. See also, U.S. Pat. No. 4,531,832, the teachings of which are also incorporated herein in their entirety, for further discussion concerning such a process.
The rapid pole transitions, for example as many as 467 per second at the sleeve surface when the magnetic core is rotated at a speed of 2000 revolutions per minute (rpm), create a highly energetic and vigorous movement of developer as it moves through the development zone. This vigorous action constantly recirculates the toner to the sleeve surface and then back to the outside of the nap to provide toner for development. This flipping action thus results in a continuous feed of fresh toner particles to the image. As described in the above-described patents, this method provides high density, high quality images at relatively high development speeds.
The above-mentioned U.S. patents, while generic to all hard magnetic materials having the properties set forth therein, prefer the hard magnetic ferrites which are compounds of barium and/or strontium, such as, BaFe12O19, SrFe12O19 and the magnetic ferrites having the formula MO.6Fe2O3, where M is barium, strontium or lead as disclosed in U.S. Pat. No. 3,716,630. While these hard ferrite carrier materials represent a substantial increase in the speed with which development can be conducted in an electrostatographic apparatus, many users of such equipment seek even faster development speeds and so further improvements to the carrier and development process are of interest.
U.S. Pat. No. 4,764,445 discloses hard magnetic ferrite carrier particles for electrographic developing applications which contain from about 1 to about 5 percent by weight of lanthanum. As mentioned in this patent, the speed of development in an electrographic process using conventional hard magnetic ferrite materials, while higher than methods using other techniques, such as with soft magnetic carriers, is limited by the resistivity of such ferrite materials. The patent discloses that addition of lanthanum to the hard magnetic ferrite crystal structure in the disclosed amounts results in a more conductive magnetic ferrite particle, yielding greater development efficiency and/or speed of development.
Others have also proposed methods for making conductive carrier particles. For example, U.S. Pat. No. 4,855,206 discloses adding neodymium, praseodymium, samarium, europium, or mixtures thereof, or a mixture of one or more of such elements and lanthanum, to a hard magnetic ferrite material to increase conductivity. U.S. Pat. No. 5,795,692 discloses a conductive carrier composition having a magnetic oxide core which is said to be coated with a layer of zinc metal that is the reaction product of zinc vapor and the magnetic oxide.
Other carriers proposed for use in an electrographic process include multi-phase ferrite composites as taught in U.S. Pat. Nos. 4,855,205; 5,061,586; 5,104,761; 5,106,714; 5,190,841; and 5,190,842.
U.S. Pat. No. 5,268,249 discloses magnetic carrier particles with a single-phase, W-type hexagonal crystal structure of the formula MFe16Me2O27 where M is strontium or barium and Me is a divalent transition metal selected from nickel, cobalt, copper, zinc, manganese, magnesium, or iron.
U.S. Pat. No. 5,532,096 discloses a carrier which has been coated on the surface thereof with a layer obtained by curing a partially hydrolyzed sol obtained from at least one alkoxide selected from the group consisting of silicon alkoxides, titanium alkoxides, aluminum alkoxides, and zirconium alkoxides. The disclosed carriers coated with such layer are said to be more durable in comparison to carriers coated with conventional resin coatings, such as those prepared using silicone, acrylic and styrene-acrylic resins.
While some of the above-described patent art may describe carriers with increased conductivity relative to traditional hard magnetic ferrite materials previously employed in development of electrostatic images, the conductivity of the carriers is believed to be so great that imaging problems are typically created due to the carrier being deposited in the image. Although not clear, it is believed that certain levels of conductivity in the carrier can facilitate a flow of charge between the carrier on the nap and the shell, thereby inducing a charge reversal on the carrier and allowing the carrier particles to electrostatically deposit on the image, referred to hereinafter as xe2x80x9cimage carrier pick-upxe2x80x9d or xe2x80x9cI-CPUxe2x80x9d. The presence of I-CPU can impact color rendition and image quality.
As can be seen, it would be desirable to develop new carriers and/or new methods for use of carriers that can be used in an electrographic process for the development of latent electrostatic images. It would also be desirable to develop carriers that can exhibit a greater level of conductivity relative to traditional hard magnetic materials previously employed in such processes, which can provide electrographic methods having higher levels of development efficiency with reduced levels of I-CPU.
The foregoing objects and advantages are realized by the present invention, which, in one aspect, concerns a method for development of an electrostatic image comprising contacting the image with a development system including at least one magnetic brush comprising:
(a) a rotating magnetic core of a pre-selected magnetic field strength,
(b) an outer nonmagnetic shell disposed about the rotating magnetic core, and
(c) an electrographic developer composition comprising (i) charged toner particles, and (ii) oppositely charged hard magnetic carrier particles with a resistivity of from about 1xc3x971010 ohm-cm to about 1xc3x97105 ohm-cm and a (Q/m)carrier of greater than about 1 xcexcC/g, the developer composition being disposed on the shell and in contact with the image,
the method resulting in a carrier deposition density on the image of less than about 0.01 g/in2.
In another aspect, the invention concerns a method for development of an electrostatic image comprising contacting the image with at least one magnetic brush comprising (a) a rotating magnetic core of a pre-selected magnetic field strength, (b) an outer nonmagnetic shell disposed about the rotating core, and (c) an electrographic developer composition disposed on the shell and in contact with the image. The developer composition comprises charged toner particles and oppositely charged carrier particles, the carrier particles comprising a hard magnetic material having a crystal structure substituted with at least one multi-valent metal of the formula Mn+, wherein n is an integer of at least 4. Preferably, the at least one multi-valent metal is selected from the group consisting of antimony, arsenic, germanium, hafnium, molybdenum, niobium, silicon, tantalum, tellurium, tin, titanium, tungsten, vanadium, zirconium, and mixtures thereof. As a matter of particular presence, the at least one multi-valent metal is selected from the group consisting of silicon, zirconium, tin, titanium, and mixtures thereof.
In a preferred embodiment, the carrier particles comprise a hard magnetic ferrite material having a single-phase hexagonal crystal structure and represented by the formula:
PFe12xe2x88x92MxO19
wherein:
P is selected from strontium, barium, or lead;
M is at least one metal selected from antimony, arsenic, germanium, hafnium, molybdenum, niobium, silicon, tantalum, tellurium, tin, titanium, tungsten, vanadium, zirconium, and mixtures thereof; and
x is less than about 0.6.
In another aspect, the invention concerns a method for development of an electrostatic image comprising contacting the image with at least one magnetic brush comprising (a) a rotating magnetic core of a pre-selected magnetic field strength, (b) an outer nonmagnetic shell disposed about the rotating core, and (c) an electrographic developer composition disposed on the shell and in contact with the image, the developer composition comprising charged toner particles and oppositely charged carrier particles. The carrier particles comprise (1) a core of a hard magnetic material having an outer surface and (2) a metal oxide composition disposed on the outer surface of the core represented by the formula MOn/2 wherein M is at least one multi-valent metal represented by Mn+, with n being an integer of at least 4. The outer surface further defines a transition zone which extends from the outer surface and into the core of the hard magnetic material where the hard magnetic material has a crystal structure within the transition zone substituted with ions of the at least one multi-valent metal ion of formula Mn+ as previously described.
In another aspect, the invention relates to a method for development of an electrostatic image comprising contacting the image with at least one magnetic brush comprising (a) a rotating magnetic core of a pre-selected magnetic field strength, (b) an outer nonmagnetic shell disposed about the rotating core, and (c) an electrographic developer composition disposed on the shell and in contact with the image, the developer composition comprising charged toner particles and oppositely charged carrier particles. The carrier particles comprise a hard magnetic ferrite material having a single-phase hexagonal crystal structure represented by the formula:
P1-yLayFe12O19
wherein:
P is selected from strontium, barium, or lead; and
y is less than 0.1.
Also disclosed are carrier particles for use in the development of electrostatic latent images, which carriers comprise the hard magnetic ferrite material substituted with lanthanum as described in the preceding paragraph.